1. Technical Field
This invention relates generally to circuits for the prevention of acoustic feedback in speakerphones, and more specifically to prevention of acoustic feedback in a speaker phone accessory for cellular telephones.
2. Background Art
Cellular telephones are becoming more and more popular. According to the Cellular Telecommunications Industry Association (CTIA), cellular telephone usage in the United States increased 27% between 1999 and 2000. As of December, 2000, there were over 109 million cellular subscribers in the United States alone.
As more people carry cellular telephones, they are increasingly using them as substitutes for conventional telephones. They talk on their cellular telephones at work and at play. There are frequent times when the person needs to use both hands for a specific task while talking on the phone. For example, business people often type on a computer while talking on the phone. In such situations, it is desirable to have a speakerphone function on the phone they are using. This presents a problem when cellular telephones are being used. While some manufacturers make cellular telephones with an on-board speaker phones, like the I1000™ manufactured by Motorola, Inc., most cellular telephones do not include speakerphone functions.
Accessory manufacturers have addressed this issue by manufacturing speakerphone attachments for cellular telephones. When the cellular telephone is inserted into the speakerphone accessory, the earpiece is disabled and an external speaker is enabled. Concurrently, the internal microphone is switched off and an external microphone is enabled. The audio signal is typically coupled to the external speaker and microphone via a communication connector at the base of the phone.
One problem that typically occurs when the phone makes the transition from earpiece to external speaker involves acoustic feedback. Cellular telephones often have a time delay in switching off the microphone. Consequently, audio energy is fed from the external speaker into the internal microphone. This feedback causes an audible “squeal” that can be heard by both parties participating in the call. The squeal is both annoying and distracting.
There is thus a need for eliminating acoustic feedback in speakerphone accessories for cellular telephones.